Field of the Invention
The present invention relates to a radiation image processing apparatus, method, and program for performing image processing based on scattered radiation on a radiation image.
Description of the Related Art
Heretofore, there has been a problem that, in case of capturing a radiation image of a subject by radiation transmitted through the subject, radiation is scattered within the subject, in particular, in case that the subject is thick, whereby scattered radiation is generated and the contrast of the radiation image is reduced by the scattered radiation. Therefore, in order to prevent a radiation detector, which detects radiation and obtains a radiation image, from being exposed to scatted radiation in case of capturing a radiation image, radiography is sometimes performed by disposing an anti-scatter grid (hereinafter, simply “grid”) between a subject and the radiation detector. As the radiography with the use of a grid causes the radiation detector less likely to be exposed to radiation scattered from the subject, the contrast of the radiation image may be improved.
The grid is formed by alternately disposing radiopaque lead or the like, and a radiolucent interspace material, such as aluminum, fiber, or the like, with a fine grid density of, for example, about 4.0 lines/mm, so that the grid is weighty. For this reason, in portable radiography performed in a patient room or the like, the grid needs to be disposed between a lying patent and the radiation detector, thereby causing a large burden of deployment work, and heavy strain on the patient at the time of radiography. Further, in the case of a converging grid, density unevenness may possibly occur due to oblique incidence of radiation. Still Further, a fine stripe pattern (moiré) corresponding to the pitch of the grid may sometimes be recorded in a radiation image together with a subject image, thereby making it hard to view the radiation image.
Consequently, it is practiced that a radiation image is captured without using a grid and an image quality improvement effect that can be obtained by removing scattered radiation using a grid is given to the radiation image through image processing, as described, for example, in U.S. Pat. No. 8,064,676 and Non-Patent Literature, C. Fivez et al., “MULTI-RESOLUTION CONTRAST AMPLIFICATION IN DIGITAL RADIOGRAPHY WITH COMPENSATION FOR SCATTERED RADIATION”, IEEE, pp. 339-342, 1996. The methods described in U.S. Pat. No. 8,064,676 and Non-Patent Literature, C. Fivez et al., “MULTI-RESOLUTION CONTRAST AMPLIFICATION IN DIGITAL RADIOGRAPHY WITH COMPENSATION FOR SCATTERED RADIATION”, IEEE, pp. 339-342, 1996 are a method that frequency-decomposes a radiation image into a plurality of frequency components, then performs scattered radiation elimination processing for eliminating contrast or latitude on a low frequency component which is regarded as a scattered radiation component, and combines the processed frequency components, thereby obtaining a radiation image eliminated of a scattered radiation component. In the method described in U.S. Pat. No. 8,064,676, the scattered radiation elimination processing is performed by multiplying a low frequency component with a gain according to the hierarchy of the low frequency component and the pixel value of the low frequency component. Here, the gain is a value less than 1 and a smaller value is allocated to a lower frequency band or a brighter pixel value. The method described in Non-Patent Literature, C. Fivez et al., “MULTI-RESOLUTION CONTRAST AMPLIFICATION IN DIGITAL RADIOGRAPHY WITH COMPENSATION FOR SCATTERED RADIATION”, IEEE, pp. 339-342, 1996 uses a table that converts a low frequency component according to the pixel value thereof and a lower frequency band is more largely suppressed in a geometric progression manner.
According to the methods described in U.S. Pat. No. 8,064,676 and Non-Patent Literature, C. Fivez et al., “MULTI-RESOLUTION CONTRAST AMPLIFICATION IN DIGITAL RADIOGRAPHY WITH COMPENSATION FOR SCATTERED RADIATION”, IEEE, pp. 339-342, 1996, strain of a patient may be alleviated at the time of radiography and image quality degradation due to density unevenness and moiré may be prevented, since no grid is required at the time of radiography.
In the meantime, it is known that, in case of capturing a radiation image of a subject by radiation transmitted through the subject, the influence of radiation scattering in the subject, reduced radiation transmission factor, and the like is increased with the increase in the thickness of the subject, and the image quality of the captured radiation image is changed. Consequently, a technology is proposed in which a body thickness which is a thickness of a subject, is roughly estimated by various types of information, such as radiography conditions and signal values of a radiation image, the width of histogram of signal values of the radiation image, or the length of the subject in a predetermined direction in the radiation image, and conditions of image processing, such as scattered radiation elimination processing to be performed on the captured radiation image and the like, or radiography conditions applied to capturing of a radiation image are changed according to the estimated body thickness.
For example, Japanese Unexamined Patent Publication No. 2(1990)-244881 discloses a method in which an association table that associates the relationship between the body thickness and the pixel value is provided by measuring pixel values of an image captured, in advance, by radiography of a simulated subject having a known thickness with known radiography conditions, then the body thickness is roughly estimated according to the pixel values of the radiation image based on the association table, and a scattered component of the radiation image is estimated according to the body thickness of the radiation image, whereby the scattered component is subtracted from the radiation image and a processed image is obtained.
In the meantime, in the radiography performed in the field of medicine, long length radiography with a long length region, such as the entire backbone (entire spine) or the entire leg (entire lower extremity) as the radiography target is sometimes performed. For radiography, various types of radiation detectors (so-called “Flat Panel Detectors”) that record a radiation image of a subject by receiving radiation transmitted through the subject have been proposed and put into practical use, but there may be a case in which the radiographable range of the radiation detector is narrower than the target desired to be radiographed. Therefore, in order to perform long length radiography, the radiation detector is moved along a predetermined movement axis so as to partially overlap and radiation transmitted through the same subject is received by the radiation detector each time the position of the detector is changed. Then, a reading operation from the radiation detector is performed with respect to each exposure of radiation (radiation image recording) and a radiation image is captured with respect to each reading operation. Thereafter, the radiation images are combined so as to be joined together, whereby a long radiation image representing a long portion of the subject is obtained.
Note that a radiation detector having a very wide radiographable area (long radiation detector, hereinafter, “long panel”) is proposed and the use of such a long panel allows a long radiation image identical to that of the long length radiography to be obtained by one radiography operation. Such long panel appears to be one panel, but a plurality of detectors is disposed inside thereof, joined in series. Then, a radiation image is obtained from each of the plurality of detectors by one exposure of radiation, and the plurality of radiation images obtained is combined with the use of software or the like, whereby a radiation image having a size larger than that of the individual radiation detectors may be obtained.
In the meantime, there may be a case in which a portion of a radiation image which is an observation target is trimmed and displayed. In such a case, image processing is performed on the trimmed area to improve the image quality of the area. For example, Japanese Unexamined Patent Publication No. 2006-068038 proposes a method in which an image processing parameter for an area to be extracted is obtained and image processing is performed on the trimmed area using the image processing parameter.